It's easy to see how low level theory supports practical development, but perhaps it is less obvious as you approach grade 4 and go on towards grades 6-8. Perhaps the reason is that nobody has actually joined the dots for me on HOW it helps - I can identify various types of cadences and pivot chords and modulations, but it doesn't get me anywhere with my practical performance that I was not already going when I didn't know the names of those things. Similarly all that stuff with learning about composers' histories. I'm sure it would be more interesting and relevant to know which country a composer was born in, where he spent his life and why he wrote that piece if I had some idea WHY it matters. To me, the fact that this quartet was written in Germany makes not a bit of difference compared to one which comes out of Italy because I have no idea why it matters. The period is useful information, but that's usually so obvious from the music that interpreting it with reference to the period is not something I think about, I don't know most of my composers' dates, but I do know how to play their music - how helpful are the dates provided you can relate them to each other and the history of the instrument and general music development?
It used to drive me barking mad to sit through the A level lessons on Sonata form and As and Bs and recapitulations and whether something is a true fugue or a proper rondo, etc. It all seemed a somewhat mechanical way of pulling apart something beautiful and enjoyable and turning it into something complex and full of rules, which had to be attacked with a pencil. Composition rules seemed particularly weird, and probably still do. There isn't exactly a "right" way of making music, from a child's little ditty to a multi-tonal symphony they are all valid.
Is there something I can read about how the piece fit together? Just now it feels like I've got a jigsaw puzzle with theory here and music history there and musical form over there and music practical there, and it doesn't go together. I wish I cared about some of this stuff, but I find it utterly dull and turgid to learn, and I'd have to be tied down and forced to read programme notes, never mind write them!